1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device comprising improved viewing angle characteristics.
2. Description of Related Art
Conventionally, liquid crystal display devices have been widely used in numerical segment type display devices such as displays of clocks and electronic calculating machines, so as to make the best use of features thereof such as thin configuration, lightness, and low power consumption.
Recently, in order to display much more information thereon, a matrix type display method has been adopted in liquid crystal display devices. In a matrix type liquid crystal display device utilizing this method, respective pixels thereof are selectively driven so as to display an image on the liquid crystal device. Liquid crystal display devices of this type have been utilized as displays of office automation apparatuses such as personal computers, word processors, copying machines etc..
Furthermore, in order to satisfy high demands for not only displaying information more densely and making a display size larger, such as five inches or more, but also displaying various kinds of information, various kinds of color display methods have been used for a color liquid crystal display device capable of displaying a color image thereon in addition to a monochromatic image.
Up to now, the following methods (a) to (d) have been proposed mainly as a display method for color liquid crystal display devices.
(a) A display method using a guest-host effect type liquid crystal display device for displaying a two-color image thereon using absorption of the incident light. In this method, dye for enabling light of different color to pass therethrough depending on an orientation direction of dye molecules is mixedly added to the liquid crystal so as to allow the orientation direction of the dye molecules to follow a change in the orientation direction of the liquid crystal, resulting in a two-color display.
(b) A display method for a color image on a liquid crystal display device constituted by combination of a twisted nematic liquid crystal cell and a color polarizer.
(c) A display method utilizing an electrically controlled birefringence type liquid crystal display device for displaying a color image thereon using a change in the birefringence of the liquid crystal according to an electric field applied thereto.
(d) A display method for displaying a color image on a liquid crystal display device comprising color filter layers of red color, green color, blue color etc. in addition to a liquid crystal layer, by utilizing the liquid crystal layer as a light shutter.
Particularly, the aforementioned display method (d) has such an advantage that a full color image can be displayed in a matrix form in a high contrast, and is one of the display methods to which attention is paid the most at present. In this display method, an active drive type twisted nematic liquid crystal display method (referred to as an active drive type TN liquid display device hereinafter), and a multiplex drive type Super Twisted Nematic liquid crystal display method are normally used (referred to as a multiplex drive type STN liquid crystal display method hereinafter). In the active drive type TN liquid crystal display method, active devices such as thin film transistors are formed as switching means for selecting respective pixels of the liquid crystal display device, and the liquid crystal is twisted at an angle of 90.degree.. On the other hand, the multiplex drive type STN liquid crystal display method utilizes steepness of light transmittance characteristics on an applied voltage obtained when a twist angle of the liquid crystal is set at an angle equal to or larger than 90.degree..
Furthermore, the active drive type TN liquid crystal display method is classified mainly into the following two methods depending on a method for arranging a pair of polarizers. One is a normally black method for displaying a black color image on a liquid crystal display device in an OFF state of a liquid crystal layer, namely, in such a state that no voltage is applied thereto. This is done arranging a pair of polarizers on both surfaces of the liquid crystal layer so that the polarization axes of the polarizers become substantially parallel to each other. Another is a normally white method for displaying a white color image on a liquid crystal display device in the aforementioned OFF state of the liquid crystal layer, by arranging a pair of polarizers on both surfaces of the liquid crystal layer so that the polarization axes of the polarizers cross each other at right angles. Particularly, the normally white method is superior to the normally black method in a display characteristics such as a display contrast, color reproductivity, and display characteristics depending on a viewing angle.
Furthermore, in the multiplex drive type STN liquid crystal display method, an optical compensation film addition type display method is mainly used, utilizing light shutter effect in a white/black display having a small dependency on the wavelength of light. The display method of this type is classified into a Double Super Twisted Nematic liquid crystal display method (referred to as a DSTN liquid crystal display method hereinafter) utilizing a liquid crystal cell as the optical compensation film wherein the liquid crystal is twisted at a twist angle in a direction opposite to that of a liquid crystal display cell, and a film addition type liquid crystal display method utilizing a film having optical anisotropy. Among those methods, it is supposed from a point of view of lightness, that the film addition type liquid crystal display method will be mainly used.
FIG. 1 is an exploded cross sectional view showing a conventional twisted nematic type liquid crystal display device 17.
As shown in FIG. 1, the liquid crystal display device 17 comprises a liquid crystal cell layer 1 containing a liquid crystal layer 13 therein. In the liquid crystal layer 13, liquid crystal molecules are arranged in a twisted nematic phase. The cell layer 1 has such a structure that the liquid crystal layer 13 is contained between a pair of optically transparent glass substrates 3 and 4 which oppose each other.
Further, optically transparent electrode films 5 and 6 for applying a voltage to the liquid crystal layer 13 are formed on respective inner surfaces of the optically transparent substrates 3 and 4, wherein the electrode films 5 and 6 are made of Indium Tin Oxide (referred to as ITO hereinafter), which is made of indium oxide to which tin is added. Further, and each of the electrode films 5 and 6 has patterns composed of plural strips which are parallel to each other so that each longitudinal direction of the strips of the optically transparent electrode film 5 solidly crosses that of the optically transparent electrode film 6 at right angles in a manner well known by those skilled in the art, resulting in plural pixels located at respective crossings in a matrix form. Furthermore, orientation membranes 7 and 8 are formed on respective inner surfaces of the electrode films 5 and 6, wherein respective surfaces of the orientation membranes 7 and 8 are previously treated by rubbing with cloth a so that the liquid crystal of the liquid crystal layer 13 is twisted at an angle of 90.degree. between the optically transparent substrates 3 and 4, as schematically shown in FIG. 1. The optically transparent substrates 3 and 4 of the cell layer 1 are bonded by a sealing element (not shown) of a resin so that the liquid crystal layer 14 is sealed therein, resulting in a completely constructed cell layer 1.
Further, on the outer surfaces of the optically transparent substrates 3 and 4, a pair of polarizers 15 and 16 are formed so that the polarization axes of the polarizers 15 and 16 cross each other at right angles as indicated by arrows 20 and 21, respectively.
Furthermore, a driving circuit (not shown) comprising a regulated voltage power source is connected to the electrode films 5 and 6, and the regulated voltage power source applies a predetermined voltage selectively to the pixels located at the crossings between respective strips of the electrode films 5 and 6 so as to change the orientation state of the liquid crystal of the liquid crystal layer 13.
For example, in the case that no voltage is applied between the electrode films 5 and 6, when light radiated from a light source is incident onto the liquid crystal display device 17 in a direction substantially perpendicular to the outer surface of the polarizer 15 as indicated by an arrow 19, only light having the polarization direction parallel to the polarization direction of the polarizer 15 as indicated by the arrow 20 is enabled by the polarizer 15 to pass therethrough. Namely, linearly polarized light having the polarization direction passes through the polarizer 15, and is incident onto the liquid crystal layer 13. Then, the polarization axis thereof is rotated at an angle of 90.degree. by the liquid crystal layer 13, the liquid crystal of which is twisted at an angle of 90.degree.. Thereafter, the linearly polarized light is incident onto the polarizer 16. As described above, the polarization direction of the polarizer 16 crosses the polarization direction 20 of the polarizer 15 at a right angle. Namely, it is perpendicular to the surface of FIG. 1 as indicated by the arrow 21. Therefore, the linearly polarized light, having passed through the cell layer 1, passes through the polarizer 16.
On the other hand, in the case that a predetermined voltage is applied between the electrode films 5 and 6, the liquid crystal of the liquid crystal layer 13 is not twisted. Therefore, the polarization direction of the light incident onto the liquid crystal layer 13 is not rotated. Then, the incident light is shaded by the polarizer 16 so as not to pass therethrough since the polarization direction of the light incident from the liquid crystal layer 13 onto the polarizer 16 is perpendicular to that of the polarizer 16 as indicated by the arrow 21.
FIG. 2 is a perspective view for defining a viewing angle .theta. which is used in the experiment of the conventional device 17 as described below and the experiment of a liquid crystal display device 18 of the preferred embodiment described in detail later.
Generally, the liquid crystal display device 17 has a specific direction 22 in which the display contrast becomes a maximum value. Above a surface 17a of the optically transparent glass substrate of the liquid crystal display device, there is defined a plane 24 including a normal 23 perpendicular to the surface 17a and extending from a point P defined on the surface 17a, and a line 22a on the surface 17a extending from the point P in a direction parallel to the direction 22 in which the display contrast becomes a maximum value. An angle at which the normal 23 crosses a line 25 of sight on the plane 24 is called a viewing angle .theta.. Namely, the viewing angle .theta. is an elevation angle at which a user watches a liquid crystal display device.
FIG. 3 shows light transmittance characteristics on an applied voltage of the liquid crystal display device 17 shown in FIG. 1, with a parameter of the viewing angle .theta., which are obtained by the measurement performed by the inventors of the present invention. In FIG. 3, a characteristic curve l1 shows the characteristic in the case of a viewing angle of 0.degree., and a characteristic curve l2 shows the characteristic in the case of any viewing angle larger than 0.degree..
As is apparent from FIG. 3, in the case of the viewing angle .theta.&gt;0.degree., the light transmittance becomes about zero % at the application of a voltage of about 2.5 V,. However, a rising phenomenon of the light transmittance is caused such that the light transmittance increases again at the application of a voltage in the range from about 3 V to about 4 V, wherein the peak value of the light transmittance is indicated by "T.theta.peak" hereinafter.
FIG. 4 shows a relationship between the viewing angle .theta. and the peak value T.theta.peak of the light transmittance of the liquid crystal display device 17.
As shown in FIG. 4, generally, the peak value T.theta.peak is about zero in the case of a viewing angle of 0.degree.. However, the peak value T.theta.peak increases as the viewing angle .theta. increases so as to have a positive correlation therebetween in the case of a viewing angle .theta.&gt;0.degree.. Due to this, in the case of a viewing angle .theta.&gt;0.degree. wherein the rising phenomenon of the light transmittance is caused, the gradation is inverted upon displaying an image on the conventional liquid crystal display device 17 since the peak value T.theta.peak of the light transmittance becomes sufficiently larger than zero %. Particularly, in a color display using a color liquid crystal display device in which it is necessary to display a color image thereon so as to have a predetermined gradation, predetermined colors of an image to be displayed cannot be displayed thereon since the peak value T.theta.peak is not zero, resulting in inversion of the gradation. Therefore, in order to obtain a liquid crystal display device having excellent light transmittance characteristics depending on the viewing angle .theta., it is necessary to cancel the peak value T.theta.peak of the light transmittance.
The applicant of the present invention proposed preferable numerical requirements of parameters for effectively canceling the aforementioned peak value T.theta.peak of the light transmittance in the Japanese patent laid open publication (JP-A) No. 1-243019/1989.
The proposed numerical requirements (a) to (c) of parameters are as follows.
(a) A retardation .DELTA.n.multidot.d which is the product of a birefringence .DELTA.n representing the anisotropy of the refractive index of the liquid crystal and a thickness d of the liquid crystal cell is preferably set so as to fall in the following range. EQU 0.3 .mu.ms.ltoreq..DELTA.n.multidot.d.ltoreq.0.6 .mu.ms, (1)
wherein
wherein .DELTA.n=n.sub.3 -n.sub.0, PA0 n.sub.e is a refractive index in a direction of the principal axis of each liquid crystal molecule contained in the liquid crystal cell, and PA0 n.sub.0 is a refractive index in a direction perpendicular to a direction of the principal axis of each liquid crystal molecule therein.
(b) A ratio K.sub.33 /K.sub.11 of a bend elastic constant K.sub.33 to a spray elastic constant K.sub.11 of the liquid crystal is preferably set at a value equal to or smaller than about 1.0.
(c) A ratio K.sub.33 /K.sub.22 of the bend elastic constant K.sub.33 to a twist elastic constant K.sub.22 of the liquid crystal is preferably set at a value equal to or larger than about 2.0.
FIG. 5 shows a relationship between the ratios K.sub.33 K.sub.11 and K.sub.33 /K.sub.22 when a composition ratio of a liquid crystal material used in a twisted nematic type liquid crystal display device is changed. In FIG. 5, ".times." represents the aforementioned relationship based on the results obtained by the measurement performed by the inventors of the present invention.
As is apparent from FIG. 5, the following characteristics similar to that known in those skilled in the art are obtained. Generally, it is understood that there is a positive correlation between ratios K.sub.33 /K.sub.11 and K.sub.33 /K.sub.22. Therefore, it is difficult to set the ratio K.sub.33 /K.sub.11 at a value equal to or smaller than 1.0 and to set the ratio K.sub.33 /K.sub.22 at a value equal to or larger than 2.0 as described above, because of the positive correlation shown in FIG. 5. Due to this, it is impossible to sufficiently decrease the peak value T.theta.peak of the light transmittance. Furthermore, in order to satisfy demand for making the display size larger, it is necessary to set the peak value T.theta.peak at a value equal to or smaller than about 3.0%, taking the visual characteristic of human being into consideration statistically.
FIG. 6 is an exploded cross sectional view of a conventional STN type liquid crystal device 26 for displaying a monochromatic image composed of white and black images by utilizing a pair of optical films 28 and 29 for canceling a phase difference between respective phases of ordinary light and extraordinary light which may generate in the liquid crystal layer. The optical films 28 and 29 for canceling the phase difference is referred to as phase difference cancellation films hereinafter. In FIG. 6, the components similar to that shown in FIG. 1 are represented by the same numerical references as that shown in FIG. 1.
In the liquid crystal display device 26, optically transparent electrode films 5 and 6 of ITO having the predetermined patterns are formed on respective inner opposing surfaces of a pair of optically transparent glass substrates 3 and 4 in the manner similar to that of the conventional liquid crystal device 17 shown in FIG. 1. Further, orientation membranes 7a and 8a are formed on respective inner surfaces of the optically transparent electrode films 5 and 6. A liquid crystal layer 27 is contained between a pair of optically transparent substrates 3 and 4, and these optically transparent substrates 3 and 4 are bonded by a sealing element (not shown) of a resin, so that the liquid crystal layer 27 is sealed therein. Respective surfaces of the orientation membranes 7a and 8a are previously treated by rubbing with cloth, so that liquid crystal of the liquid crustal layer 27 is twisted at an angle in the range between 180.degree. and 260.degree., namely, liquid crystal molecules thereof are arranged in a super twisted nematic phase.
Further, on respective outer surfaces of the optically transparent substrates 3 and 4, there are formed a pair of phase difference cancellation films 28 and 29 of a uniaxially drawn polymeric film having a predetermined optical axis. Furthermore, on respective outer surfaces of the phase difference cancellation films 28 and 29, a pair of polarizers 15 and 16 are formed in the manner similar to that of the conventional liquid crystal display device 17 shown in FIG. 1, resulting in the STN liquid crystal display device 26.
Normally, in the STN type liquid crystal display device, the birefringence thereof becomes relatively large by the aforementioned super twist of the liquid crystal, and incident light having passed through the liquid crystal display device is colored with a yellow green color in a so-called yellow green mode, or with a blue color in a so-called blue mode. In order to prevent the incident light from being colored so as to improve the visibility, there has been developed the STN type liquid crystal display device for displaying a monochromatic image composed of white and black images thereon by arranging a pair of optical phase difference cancellation films 28 and 29.
The STN type liquid crystal display device 26 for displaying a monochromatic image thereon by utilizing the optical phase difference cancellation films 28 and 29 has a relatively large display capacity since it is capable of being driven using a time division method at a relatively high frequency. Further, it is capable of displaying a monochromatic image composed of white and black images thereon in a relatively high contrast, resulting in a distinct display. Furthermore, since the STN type liquid crystal display device 26 becomes capable of displaying a color image thereon by arranging color filter layers, it has been utilized as displays of personal computers, word processors etc.. However, since the STN type liquid crystal display device 26 has a relatively large dependency of light transmittance on the viewing angle 8 which is the elevation angle at which the user watches the liquid crystal display device 26, it has such a disadvantage that viewing angle characteristics thereof is inferior to the other type liquid crystal display devices.